1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens which is preferable for use in an image-taking apparatus such as a video camera and a digital camera which uses a solid-state image pickup element as an image pickup device.
2. Description of the Related Art
Conventionally, a zoom lens of a so-called negative lead type having a lens unit with a negative optical power at the front has been used as a standard zoom lens in many cameras since it relatively easily achieves image taking at a wider angle.
As an exemplary standard zoom lens of this type, Japanese Patent Application Laid-Open No. 1978-132360 (corresponding to U.S. Pat. No. No.4,299,452), Japanese Patent Application Laid-Open No. 1981-19022 (corresponding to U.S. Pat. No. 4,370,031), and U.S. Pat. No. 5,283,639 each have proposed or disclosed a so-called two unit zoom lens consisting of two lens units, that is, a first lens unit having a negative optical power and a second lens unit having a positive optical power, in which these two lens units are moved along an optical axis to change the spacing between the lens units to provide zooming.
Japanese Patent Application Laid-Open No. 1995-52256 has proposed a zoom lens having, in order from an object side, a first lens unit having a negative optical power, a second lens unit having a positive optical power, and a third lens unit having a positive optical power, in which the spacing between the second lens unit and the third lens unit is increased to provide zooming from the wide angle end to the telephoto end.
In addition, U.S. Pat. No. 5,434,710 has disclosed a zoom lens having, in order from an object side, three lens units consisting of a first lens unit having a negative optical power, a second lens unit having a positive optical power, and a third lens unit having a positive optical power, in which the spacing between the second lens unit and the third lens unit is reduced to provide zooming from the wide angle end to the telephoto end.
The present inventor has also proposed a multiunit zoom lens comprising of three or more lens units in Japanese Patent Application Laid-Open No. 1994-27377 (corresponding to U.S. Pat. No. 6,104,548).
Typically, the negative lead type zoom lens having a lens unit with a negative optical power at the front is characterized in that image taking is relatively easily performed at a wider angle and that a predetermined back focal distance can be readily provided.
To achieve favorable optical performance throughout the zoom range or throughout an image plane, it is necessary to appropriately set the arrangement of the optical powers of respective lens units and the shapes of lenses.
Inappropriately setting the arrangement of the optical powers of the respective lens units and the lens shapes increases variations in aberration associated with to cause difficulty in providing high optical performance throughout the zoom range.
Especially, in the two unit zoom lens having a lens unit with a negative optical power at the front, the relative positions of the respective lens units on an optical axis are uniquely determined resulting from varied magnification and corrected variations in the position of an image plane. As a result, optical performance cannot be controlled arbitrarily at a certain position while zooming from the wide angle end to the telephoto end.
Variations in aberration need to be minimized in the respective lens units during zooming in order to provide favorable optical performance at a certain point during zooming. As a method to address this, for example, the optical power is reduced in each lens unit, or a larger number of lens elements are used to form each lens unit. These methods, however, lead to an increase in the overall length of the lenses and difficulty in achieving high zooming ratio and high performance.
To solve the problems, U.S. Pat. No. 5,570,233 has disclosed a zoom lens consisting of, in order from an object side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power, in which each lens unit is moved to provide zooming.
With the development of the image pickup device, however, further improvement in optical performance is required in the field of optical apparatuses such as a video camera and a digital camera in which increasingly higher performance is desired.
Additionally, in a zoom lens of a type in which a lens unit closer to an image plane than an aperture stop moves in association with zooming, the exit pupil distance largely varies as a higher zooming ratio is provided. In a CCD frequently used as an image pickup device in recent years, for example, if the incident angle of light rays on an imaging plane varies, the angle of the light rays passing through a color filter is changed to create the possibility of causing a problem of color bleeding on the periphery of an image.
It is an object of the present invention to provide a zoom lens comprising four lens units which achieves a high zoom ratio and provides favorable optical performance at an arbitrary zoom position.
It is another object of the present invention to provide a zoom lens which involves small variations in the exit pupil distance during zooming, in addition to the aforementioned object.
To achieve the aforementioned objects, the zoom lens according to an aspect of the present invention includes, in order from an object side to an image side, a first lent unit which has a positive optical power, a second lens unit which has a negative optical power, a third lens unit which has a positive optical power, and a fourth lens unit which has a positive optical power. During zooming from the wide angle end to the telephoto end, the spacing between the first lens unit and the second lens unit is increased, the spacing between the second lens unit and the third lens unit is reduced, the spacing between the third lens unit and the fourth lens unit is increased, and an aperture stop moves together with the third lens unit. In addition, specific conditions described in the following embodiments are satisfied.
According to another aspect of the present invention, the zoom lens includes, in order from an object side to an image side, a first lens unit which has a positive optical power, a second lens unit which has a negative optical power, a third lens unit which has a positive optical power, and a fourth lens unit which has a positive optical power. The spacing between the respective lens units changes during zooming, and specific conditions described in the following embodiments are satisfied.
Characteristics of the zoom lens of the present invention will be apparent from the following description of specific embodiments with reference to the drawings.